The present invention relates to a print head that is employed as a exposure light source for forming an electrostatic latent image on a photosensitive body in a printing apparatuses such as printers and facsimile machines adopting the electrophotographic printing system, and the present invention also relates to a matrix-type light emitting element array chip to constitute the print head and a light emitting element array drive IC that drives the matrix-type light emitting element array chip.
Print heads that are employed as an exposure light source for a photosensitive body in a printing apparatus adopting the electrophotographic printing system in the prior art include those that use as a light source, a light emitting element array constituted by providing a number of light emitting elements such as semiconductor laser elements, LED elements and the like in a single row, and those that use a fluorescent light, a cold-cathode tube or the like as a light source and use a liquid crystal element as a shutter. Those different types of print heads all selectively expose the photosensitive body that has been uniformly charged by a charger with the light source to form an electrostatic latent image on the photosensitive body.
In this document, a light emitting element array achieved by providing a great number of LED elements is referred to as an LED array, and a print head constituted with an LED array is referred to as an LED print head. An LED print head may be constituted by mounting on a mounting substrate LED array chips each achieved by providing a great number of LED elements on a single semiconductor chip and light emitting element array drive ICs each provided for controlling light emission of the individual LED elements in each of the LED array chips, in quantities that correspond to the printing width.
FIG. 8 is a top view illustrating the structure of an LED array chip in the prior art. The LED array chip 51 illustrated in FIG. 8 is provided with LED elements 52 provided in a single row, individual pad electrodes to be driven 53 that are individually connected to the anodes of the LED elements 52 and a common electrode 54 that is connected to the cathodes of all the LED elements 52. The common electrode 54 is provided at the rear surface of the LED array chip 51. In addition, FIG. 9 is a partial enlargement of an LED print head in the prior art mounted with the LED array chip illustrated in FIG. 8. The LED print pad illustrated in FIG. 9 is constituted by mounting the LED array chip 51 and an LED array drive IC 55 for driving the LED array chip 51 at a mounting substrate 59. The individual pad electrodes to be driven 53 of the LED array chip 51 and drive pad electrodes 60 of the LED array drive IC 55 are connected with each other through metal wires 56, and I/O pad electrodes 61 of the LED array drive IC 55 are connected with wiring patterns (conductive patterns) 62 for control signals and the source formed at the mounting substrate 59. The common electrode 54 formed at the rear surface of the LED array chip 51 is connected to a cathode wiring pattern (conductive pattern) 58 formed at the mounting substrate 59.
Individual sets of data corresponding to the LED elements 52 at the LED array chip 51, a strobe signal for controlling the light emission time for the LED elements and the like are input through the LED array drive IC 55. The LED array drive IC 55 is provided with a shift register that stores individual sets of data that have been input, drive current source circuits each of which outputs a drive current corresponding to one of the individual sets of data and switching circuits that turn ON/OFF between the output terminals of the drive source circuit and the drive pad electrodes 60 in conformance with the strobe signal that has been input. The number of shift register stages, the number of drive current source circuits, the number of switching circuits and the number of drive pad electrodes 60 all correspond to the number of LED elements 52 at the LED array chip 51. In the LED array drive IC 55, a drive current is output by, for instance, the drive current source circuit corresponding to the printing data that indicate light emission, and with the switching circuits set to ON while the strobe signal is at "H" level, the drive current is supplied to the LED elements 52 that correspond to the printing data at "L" level via the individual pad electrodes 53 to be driven to achieve selective light emission. Thus, in the LED print head of the prior art, the anodes of the LED elements 52 at the LED array chip 51 are connected to the individual pad electrodes 53 to be driven (the drive pad electrodes 60 at the drive IC 55) on a one-to-one basis.
However, as the high density of the LED elements at an LED array chip increases (achieving a dot density of 600 DPI and an LED element pitch of 42.3 micrometer, for instance), it becomes increasingly difficult to individually connect the individual pad electrodes to be driven 53 and the drive pad electrodes 60 that are provided in correspondence to LED elements 52 on a one-to-one basis through wire bonding. A matrix-type LED array chip, such as that disclosed in Japanese Unexamined Patent Publication No. 1987-152873 provides a solution to this problem.
In a matrix-type LED array chip, matrix-connections are achieved between M.times.N light emitting elements and M pad electrodes to be scanned connected to the cathodes of the light emitting elements and between the light emitting elements and N pad electrodes to be driven connected to the anodes of the light emitting elements, in order to reduce the number of pad electrodes in the LED array chip. A matrix-type LED array chip may assume a structure, for instance, in which, a semiconductor chip is divided into M semiconductor blocks, a pad electrode to be scanned and N light emitting elements are formed on each semiconductor block and N pad electrodes to be driven are formed on the M semiconductor blocks, with the cathodes of the N LED elements within each block connected with the pad electrode to be scanned within the block and the anodes of the N LED elements within the block individually connected with different pad electrodes to be driven.
However, in a print head employing the matrix-type light emitting element array chip in the prior art described above, it is necessary to mount two types of drive circuits (drive ICs), i.e., a data control circuit (data control IC) that controls the pad electrodes to be driven in correspondence to individual sets of data and a scan control circuit (scan control IC) for scanning the pad electrodes to be scanned at a mounting substrate and to bond between the pad electrodes to be driven and the data control circuit and between the pad electrodes to be scanned and the data control circuit with metal wires, and this poses an obstacle in achieving a reduction in the size of the print head. It also presents a stumbling block in achieving a reduction in the number of manufacturing steps for manufacturing the print head and a reduction in the manufacturing cost. Since, for instance, a structure in which a light emitting element array chip constituted by providing the pad electrodes to be driven and the pad electrodes to be scanned on either side of the row of light emitting elements is used, a data control circuit and a scan control circuit are mounted at the sides of the light emitting element array chip and the metal wires are drawn out from the two sides of the light emitting element array chip, must be adopted, the size of the print head cannot be reduced and the number of wire bonding steps cannot be reduced either.